1. Field of the Invention
This invention relates to the purification of methacrylamidopropyltrimethylammonium chloride (sometimes abbreviated herein as MAPTAC). More particularly, this invention relates to the use of a microporous crystalline silica for removing trace quantities of detrimental impurities from methylacrylamidopropyltrimethylammonium chloride.
2. Prior Art
Methylacrylamidopropyltrimethylammonium chloride is a cationic vinyl monomer that undergoes homo- and copolymerization to provide polymeric products containing pendant quaternary ammonium groups that are useful, for example, in liquid/solid separation processes relating to water pollution control, paper manufacturing, textile finishing, etc. However, the properties of the homopolymers and copolymers prepared from methacrylamidopropyltrimethylammonium chloride are adversely affected by the presence of even trace quantities of impurities such as allyl methacrylate, methacrylic acid, diphenyl amines, etc. that are formed or introduced during the synthesis of this material. For example, as little as ten parts per million of allylmethacrylate or 15 parts per million of the methyl ether of hydroquinone will adversely affect the properties of the polymers made from methacrylamidopropyltrimethylammonium cyloride.
Microporous crystalline silica is a known commerical product having a topological type of tetrahedral framework characterized by straight channels along a b-axis defined by 10 membered oxygen rings with an elliptical cross section of 5.7-5.8 .ANG. by 5.1-5.2 .ANG.. These channels are interconnected by zigzag channels along an a-axis also defined by 10 membered oxygen rings with a nearly circular cross section of 5.4.+-.0.2 .ANG.. These channels are accessible to molecules having a diameter up to 6 .ANG.. A commercial material is available from Union Carbide Corporation under the tradename "Silicalite". The properties and characteristics of the microporous crystalline silica which is a hydrophobic/organophilic silica, are described in greater detail in a paper entitled "Adsorption and Diffusion of C.sub.6 and C.sub.8 Hydrocarbons in Silicalite" by Pingdong Wu et al. presented at the AIChE Orlando, Fla., Meeting of Feb. 28-Mar. 3, 1982.
Although methacrylamidopropyltrimethylammonium chloride can be purified by conventional techniques such as solvent extraction and crystallation, it can be done so only with appreciable difficulty requiring multiple processing steps and significant processing equipment.
It is also suggested in U.S. Pat. No. 3,907,891 to Guilbault et al. dated Sept. 23, 1975 that methacrylamidopropyltrimethylammonium chloride, in aqueous solution, can be purified by contacting the solution with activated carbon. Although this process gives reasonably satisfactory results, there is still need for improvement in purifying methacrylamidopropyltrimethylammonium chloride in aqueous solution. For example, comparatively high temperatures are required to regenerate the activated carbon. Also, the carbon particles tend to crush which can result in the plugging of flow lines and/or the presence of very fine carbon particles in the purified product.